Dielectric waveguides

ABSTRACT

A dielectric waveguide for millimetric wavelengths comprising a core of polymer material supported by a body of foamed polymeric material and enclosed in a protective jacket.

KR '3a703s690 United States Patent Ravenscroft et al.

rag/a1 5] Nov. 21, 1972 DIELECTRIC WAVEGUIDES Inventors: lvor AlbertRavenscrolt, Woodbridge; Lynden Ashrooke Jackson, Ipswich, both ofEngland Assignee: The Post Office, London, England Filed: Dec. 15, 1970Appl. No.: 98,242

Foreign Application Priority Data Dec. 17, 1969 Great Britain..6l,589/69 US. Cl. ..333/95 5, 333/98 M, 350/96 Int. Cl. ..H0lp 3/16,HOlp 3/18 Field of Search ..333/95, 95 S, 98, 98 M References CitedUNITED STATES PATENTS 11/1964 Hicks, Jr. et al ..333/95 X 3,040,2786/1962 Griemsmann ..333/95 2,769,148 10/1956 Clogston ..333/95 X2,794,959 6/1957 Fox ..333/95 X 3,386,787 6/1968 Kaplan ..333/95 X3,434,774 3/1969 Miller ..333/95 X 3,542,536 11/1970 PrimaryExaminer-l-lerman Karl Saalbach Assistant Examiner-Marvin NussbaumAttorney-Hall & Houghton ABSTRACT A dielectric waveguide for millimetricwavelengths comprising a core of polymer material supported by a body offoamed polymeric material and enclosed in a protective jacket.

4 Claims, 7 Drawing Figures OR IN; 333795 1 p Flam et al. ..333/95 XP'A'TENTEDnuvz: m2

sum 1 nr 6 FIG. 2.

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I 1 01? A PA VE/I/SCROFT INVENTOR ATTORNEY P'A'TE'NTEDInm I972 SHEET 60F 6 Ecimg z kqbzmtq INVENTORS DIELECTRIC WAVEGUIDES BACKGROUND OF THEINVENTION It has been proposed to employ a rod of dielectric material asa waveguide but in the past low loss transmission has been achieved onlywith rods of diameter small in relation to wavelength. Dielectric rodsof such small diameter do not possess adequate guiding properties andtherefore have to be employed in straight lengths which limits theirusefulness.

It is an object of the present invention to provide an improveddielectric rod waveguide.

SUMMARY OF THE INVENTION According to the present invention a dielectricrod waveguide comprises a core of a polymeric material of a loss angleabout 50 microradians or less and of a diameter from about 0.5 A a toabout 0.85 A where A, is the free space wavelength of theelectromagnetic energy to be transmitted along the waveguide, the corebeing supported by a body of polymeric material in foamed form, havingan effective dielectric constant substantially equal to unity and of asize such that the overall diameter of the core and support body is of avalue from about 1.6 A, to about 5.0 A,,, the whole lying within ajacket providing protection against mechanical stresses, the ingress ofmoisture and providing electrical screening.

The core may be composed of polypropylene, a particularly suitable formof which is that known as PXC 3391 manufactured by Imperial ChemicalIndustries Limited.

The polymeric material in foamed form supporting the core may be thesame material as that of which the core is composed.

Preferably, the loss angle should be below microradians.

The core may be in the form of a solid rod or it may consist of a numberof strands of the dielectric material or it may be a rod of foameddielectric material. Both these alternative forms have a lower densitythan the solid rod and both have a lower effective dielectric constantand lower effective loss angle.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an enlarged cross-section ofthe waveguide,

FIG. 2 shows on a difierent scale from FIG. 1 and in diagrammatic formthe field configuration of the HE mode in the waveguide of FIG. 1, and,

FIGS. 3, 4, 5, 6 and 7 are graphs illustrating various relationships.

DESCRIPTION OF PREFERRED EMBODIMENTS The waveguide shown in FIG. 1consists of a core 1 comprising a solid rod of polypropylene ofelectrical grade having a dielectric constant e, 2.26 of a loss anglesequal to 50 microradians. The diameter of the core 1 is 5.3 mm and it iscovered by a sheath 2 of low-loss foamed polypropylene preferably of thesame grade as that from which the core is made. The overall diameter ofthe core 1 and sheath 2 is 28 mm. The outer surface of the sheath iscovered with a layer 3 of a loss'y" foam having the same effectivedielectric constant as the low loss foam. The final covering of thewaveguide is a protective jacket 4 of a suitable polymer. The jacket 4pro- 7 vides protection against mechanical stresses and electricalscreening. Embedded in or otherwise forming part of the jacket is awater barrier providing adequate protection against ingress of moisture.

The waveguide supports the HE mode and operates over the frequency range29-39 Gl-Iz. The field configuration of the HE. mode is indicated inFIG. 2.

The core 1 may be formed by extrusion, care being taken to the diameterof the core 1 to within very close tolerances about i 1 percent and toensure that the core is totally homogeneous and of a high qualitysurface finish. The core 1 may be extruded by a method in which moltenmaterial is extruded through a nozzle along whose length the rate ofcooling of material passing through it is varied in such manner as toperrnit the application to the molten material of an extra or back-uppressure which substantially prevents the appearance of voids. Thenozzle has a bore which has an extremely smooth surface thereby ensuringthe requisite high quality surface finish for the core.

The sheath 2 may be made by conventional foaming techniques and may beapplied to the core during extrusion of the latter or after. The core 1may be passed into a chamber inside which the core is held at atemperature below the softening of the material from which the core ismade, sheathing material being fed to the chamber and therein bonded tothe core. The sheathing material may be allowed to foam or be foamedafter entry into the chamber or the sheathing material may be fed infoamed form into the chamber.

Application of the lossy material comprising the layer 3 may be effectedby the introduction of a substance having a high loss angle, for examplecarbon, into the outer regions of the foamed sheath 2, or alternatively,the lossy material may be introduced as part of a separately extrudedlayer.

The group velocity of the HE mode depends to a large extent on thediameter in wavelengths of the core 1, and varies between limitsobtained by propagation in free space, when the core diameter tends tozero, and by propagation entirely through the material of the core. FIG.3 shows, for material of dielectric constant e, 2.26, the relationshipbetween group velocity and normalized frequency (core diameter D/freespace wavelength) obtained by numerical methods. If dispersion is to bekept at acceptably low values, rod diameters below about 0.3 A, and fromabout 0.5 A and above must be used, say from about 0.5 A, to 0.85 A theupper limit being reasonably close to the cut-off point for higher modesof operation of the waveguide.

The relationship between group velocity and frequency for the core 1 ofthe waveguide of FIG. 1 is shown in FIG. 4.

Since the core 1 must be supported in some suitable manner, practicalvalues for the extent of the radial field of the I-IE, mode must beascertained. To indicate the extent of the radial field, the graph ofFIG. 5 has been prepared showing the diameter of D1 1A,, of theconcentric cylinder through which 99 percent and 99.99 percent of thetotal power in the propagated wave flows. This indicates that thevariation of the diameter of percentage power flow with frequency isfairly uniform over the range D X 0.5 A, and 0.72 A

FIG. 5 also shows that 99 percent of the total power is transmittedthrough a cylinder of a diameter D1 of about 1.6 A over the range of Djust mentioned whilst 99.9 percent of the total power is transmittedthrough a cylinder of a diameter D1 of about 1.4 A However, if thelatter value is increased to about 5.0 A a downward extension oftransmitted bandwidth is possible.

FIG. 6 shows the relationship between extent of radial field andfrequency of operation for the core 1 of the waveguide of FIG. 1. FIG. 6shows that the overall diameter of the cylinder containing 99.99 percentof the energy is within 28 mm over the frequency range 29-39 GHz.

The attenuation of the HE mode is given approximately by the equation ag 1 nepers/metre where A, free space wavelength in meters 6 dielectricconstant of core 1 to sheath 2 -y loss angle of core 1 in radians N /Nmillimetric of power inside rod to the total power transmitted.

FIG. 7 shows the variation of the attenuation of the waveguide of FIG. 1with frequency over the range 29-39 GHz.

Thus, by minimizing losses in the material, attenuation, for a given setof operating parameters, is reduced. The dielectric loss may also bereduced by adopting a core of foamed or standard form.

We claim:

1. A dielectric rod waveguide for the transmission of electromagneticenergy at frequencies in the millimetric waveband, the waveguidecomprising in combination a core through which a proportion but not allof said electromagnetic energy is transmitted, said core having adiameter lying within the range from about 0.5 A to about 0.85 A where Ais the free space wavelength of the electromagnetic energy to betransmitted and being of a polymeric material with a loss angle notgreater than about 50 microradians, a core support body round the corethrough which substantially the balance of said electromagnetic energyis transmitted, the support body comprising a tubular body of polymericmaterial in foamed form and having a dielectric constant ofsubstantially equal'to unity and of a size such that the overalldiameter of core and support body lies within the range from about 1.6A, to about 5.0 A lossy material associated with said support body toabsorb electromagnetic energy incident upon it, and, surrounding thecore support, a jacket providing protection against mechanical stresses,the ingress of moisture and electrical screening.

2. A dielectric rod waveguide as claimed in claim 1 in which the core isof stranded form.

3. A dielectric rod waveguide as claimed in claim 1 in which the core isof foamed form.

4. A dielectric rod waveguide as claimed in claim 1 in which thepolymeric material of which the core is formed is the same as that ofwhich the core support is formed.

1. A dielectric rod waveguide for the transmission of electromagnetic energy at frequencies in the millimetric waveband, the waveguide comprising in combination a core through which a proportion but not all of said electromagnetic energy is transmitted, said core having a diameter lying within the range from about 0.5 lambda o to about 0.85 lambda o where lambda o is the free space wavelength of the electromagnetic energy to be transmitted and being of a polymeric material with a loss angle not greater than about 50 microradians, a core support body round the core through which substantially the balance of said electromagnetic energy is transmitted, the support body comprising a tubular body of polymeric material in foamed form and having a dielectric constant of substantially equal to unity and of a size such that the overall diameter of core and support body lies within the range from about 1.6 lambda o to about 5.0 lambda o, lossy material associated with said support body to absorb electromagnetic energy incident upon it, and, surrounding the core support, a jacket providing protection against mechanical stresses, the ingress of moisture and electrical screening.
 1. A dielectric rod waveguide for the transmission of electromagnetic energy at frequencies in the millimetric waveband, the waveguide comprising in combination a core through which a proportion but not all of said electromagnetic energy is transmitted, said core having a diameter lying within the range from about 0.5 lambda o to about 0.85 lambda o where lambda o is the free space wavelength of the electromagnetic energy to be transmitted and being of a polymeric material with a loss angle not greater than about 50 microradians, a core support body round the core through which substantially the balance of said electromagnetic energy is transmitted, the support body comprising a tubular body of polymeric material in foamed form and having a dielectric constant of substantially equal to unity and of a size such that the overall diameter of core and support body lies within the range from about 1.6 lambda o to about 5.0 lambda o, lossy material associated with said support body to absorb electromagnetic energy incident upon it, and, surrounding the core support, a jacket providing protection against mechanical stresses, the ingress of moisture and electrical screening.
 2. A dielectric rod waveguide as claimed in claim 1 in which the core is of stranded form.
 3. A dielectric rod waveguide as claimed in claim 1 in which the core is of foamed form. 